Method and apparatus for testing paperboard



R. C. M KEE ET AL METHOD AND APPARATUS FOR TESTING PAPERBOARD v May 12, 1953 2 Sheets-Sheet 1 Filed Sept. 9, 1948 y ,1953 R, c. MGKEE ET AL 2,537,996

METHOD AND APPARATUS FOR TESTING PAPERBOARD Filed Sept. 9, 1948 2 Sheets-Sheet 2 Patented May 12, 1953 METHOD AND APPARATUS FOR TESTING PAPERBOARD Robert C. McKee and George R. Sears, Appleton,

Wis., assignors to The Institute of Paper Chemistry, Appleton, Wis., a corporation of Wisconsin Application September 9, 1948, Serial No. 48,375

6 Claims. 1

The present invention relates to a method and apparatus for testing paperboard, and in particular relates to the evaluation of the properties of paperboard known as the resistance to tear and the resistance to continued tearing.

During the handling and the shipment of assembled and packed paperboard containers, the paperboard is subjected to tearing along the surfaces thereof and along the score lines which are provided to facilitate setting up the assembled containers from blanks. For example, in the case of containers packed with cans, the edges or chines of the cans tend to cut into the paperboard forming the container, and sometimes continued handling causes punctures or cuts through the walls of the container and through the score lines. The cuts, which result from the movement of the cans, grow larger with continued movement and the paperboard gradually tears until adjacent cuts meet and form long tears which may extend the entire length of the box. In most instances the resistance to tear is extremely important, especially in containers which are to be shipped long distances or which are adapted to contain sharp objects. However, this property does not adapt itself to evaluation by ordinary testing methods and apparatus.

Heretofore the ability of a paperboard shipping container to resist tearing has been evaluated indirectly by the correlation of other box and 1 material properties, as for example, the resistance of an assembled box to crushing and to impact, and the resistance of the component materials to penetration, to compression, and to bursting. While the data obtained from these varied tests gives some indication of the overall tearing strength of the container, itdoes not result in a positive index of the resistance of paperboard materials and the assembled paperboard containers to tearing.

Various testing apparatuses have been evolved for measuring the tearing strength of ordinary paper sheets but these instruments are not applicable to the testing of paperboard or corrugated paperboard because they do not produce a tear which is straight and symmetrical. In addition, when the prior art tests are used, corrugated board and paperboard tend to delaminate, thus giving test results which are not reproducible. Accordingly, the principal object of the present invention is the provision of a method and apparatus for evaluating the tearing strength and the resistance to continued tear of paperboard and like materials. A more specific object of the invention is the provision of apparatus for measuring the tearing properties of paperboard and the like, the apparatus producing a relatively straight tear without de-laminating the paperboard, thereby resulting in test data which are direct measures of the tearing strength and the resistance to continued tear. Other objects and advantages of the invention will become clear from the following description of one preferred embodiment of the apparatus and the accompanying drawings thereof.

In the drawings:

Fig. l is a fragmentary, perspective view of the tear testing apparatus having various of the features of the invention; 7 v

Fig. 2 is an elevationa1 view, partially in section, showing the apparatus having a test specimen clamped therein preparatory to making a test;

Fig. 3 is a sectional view taken on line 3-3 in Fig. 2;

Fig. 4 is a view similar to Fig. 2 showing the specimen and the apparatus during test;

Fig. 5 is an enlarged diagrammatic view showing the tearing action of the tester;

Fig. 6 is a fragmentary perspective view of a preferred form of test specimen for use in testing corrugated paperboard; and

Fig. 7 is a fragmentary, side elevational view showing the means employed for indicating theenergy required to tear the test specimen.

The general method of tear testing samples of paperboard in accordance with the invention, comprises clamping the samples to be tested along spaced apart lines which are parallel to, and which are symmetrically spaced relative to, the line along which the tearing strength of the sampie is to be determined. The clamped portions are then rotated relative to one another about an axis which is normal to the line along which the tear is to be produced, which bisects the length of the specimen along the proposed line of the tear, and which passes through the center of the specimen, that is, the axis falls midway between the outer plane surfaces of the specimen. In order to secure reproducible results and to test the property of continued tear rather than initial tear, the sample, before testing, is cut inwardly for a short distance at each end of the proposed line of tear, and the center of the sample is cut away to leave a symmetrically disposed opening.

A typical test specimen 9 for use with the embodiment of the testing apparatus to be described is illustrated in Fig. 6. The specimen 9 is a piece of corrugated paperboard which comprises a pair of spaced apart layers of paperboard i l which are held spaced apart and which are mechanically interconnected by a sinuous spacer sheet 13. The line along which the tearing strength is to be tested is indicated by the dotted line i5 and the axis of rotation of the half sections of the sample is indicated as line H. As illustrated, the proposed line of tear is perpendicular to the corrugations and to the axis I! about which the half sections are to be rotated, however, if it is desired to test the resistance to tear along the corrugations, the axis of rotation should extend perpendicular to the corrugations. The'line of tear will then extend parallel'to the corrugations. The test specimen 9 is cut into the shape of a rectangle which is symmetrical about the proposed line of tear. As before pointed out, a knife cut I?) of short length is made at each end of the line of tear i 5 so that the test is a measure of continued tear. At the mid-point of the line of tear 45, a geometrically symmetrical figure is cut from the center of thesample to avoid the inaccuracies observed when the sample is tornadjacent the center of rotation of the half sections.

Conveniently, as illustrated, the symmetrical opening or cut out portion 2.! may be in the shape of a square.

In carrying out numerous tests upon paperboard specimens of all types with the apparatus which is to be described, it has been determined that a particularly convenient size for the test specimen is a rectangle 2 inches by 5 inches having a square cut out portion in the center thereof which has a diagonal measurement of 0.78 inch. proposed line of tear so that the tearing will be a measure of the continued tear rather than the initial tear. The uncut length of the specimen between the end of each knife cut and the nearest corner of the center hole in the specimen is 1.85 inches. Since the specimen is subsequently torn symmetrically from both ends (see Fig. 5). the total length of tear is 3.70 inches.

The apparatus of the invention which is used to carry out the above tearing test includes a main frame 23, a fixed clamp and a rotatable clamp 27 which are-spaced apart and which are parallel to one another, means for rotating the rotatable clamp 21, and means for measuring the amount of energy required to effect the tearing of the specimen. In the illustrated embodiment of the apparatus, the fixed and the rotatable clamps, 25 and 2?. respectively, and the other elements are supported upon the frame 23 which is fabricated from pipe or the like. The frame 23 is provided with four vertically extending standards 29 which are adapted to be attached to the T1001 of the testing laboratory. The standards thereby provide a stationary base for the apparatus. The four standards 29 are interconnected at their upper ends by suitable cross members 3| to provide a support for the clamps and for the'indicating device and to reinforce the structure.

Each of the clamps 25 and 2 include a-main support member 33 having a U-shaped crosssection, as illustrated in Fig. 3. One of' the arms of each of the U-shaped main support members 33 includes a clamping jaw 35 which is movable.

to grip the specimen 9. The other of the arms of each of the U-shaped support members 33 is provided with an adjustablypositionable clamping jaw 39 for accommodating samples of varying thicknesses. By properly adjusting the jaws 3,5 and 39,, it is possible to position the sample Knife cuts are made at each end of the I 9 so that its center coincides with the axis of rotation IT! of the rotatable clamp 21 and so that the axis of rotation passes midway between the outer surfaces of the test specimen. The adjustable jaw 33 may be positioned by set screws 4| and shims 53 as illustrated, or by any other convenient positioning means.

The clamping jaw 25 is made movable in the structure illustrated by the provision of a pair of adjusting screws which are egaged in the associated arm of the U-shaped clamp member 33; The inner end of each of the adjusting screws 45 is provided with a portion of reduced diameter 4'1 which includes a bearing surface 49 adapted tobe rotatably engaged in the movable jaw member 35. Loosening the screws 415 permits the removal of the sample under test and a new sample may be inserted between the jaws 39 and 35 and thcmovable jaw 35 retightened to grip the sample for test.

If tearing tests on paperboards having varying thicknesses are to b made during a single test period, it is desirable to provide each of the adjustably positionable clamping jaws 39 witha screw adjustment similar to that described in connection with the jaws 35. This enables the operator to set up the apparatus for any thickness of paperboard on short notice. The rate of testing samples may be increased with the arrangement. described, by providing a series of shims it which are marked with code designationsv for the types of paperboard being tested.

The upper end of the fixed clamp 25 is rigidly afiixed to one of the cross members am of theframe 23, as illustrated. The faces of the jaws 35 and. 39 associated with the fixed clamp 25 are vertically extending and are disposed in a direction extending across the apparatus. 7

The rotatable clamp 27-is provided with a stub shaft 5!. which is adapted to be rotatably supported in a pair of spaced apart low-friction bearings 53. The bearings 53 are supported in a pair of depending members 55-whose upper ends are attached to the cross brace am. The bearings 53- are positioned so that the axis of the shaft 5| passes between the clamping faces of the jaws 35 and 39 on each of the clamps 25 and 21. The clamp 2". may thus be rotated from a position in which the jaws on each of the clamps 25. and

2.7 lie in common planes through an angle of, sub-- stantially degrees as will be hereinafter set forth. Thev adjacent edges 56 and 56a. of the clamping jaws-35 and 39, respectively, of the clamping members 25and 2! are all parallel and extend normal to the axis of rotation of the shaft 5!, the axis of the-shaft 5| when extended, coinciding with the axis of rotation ll of the sample 9. The arrangement of the faces and edges of the jaws thereby causes the two clamps of the apparatus to grip the sample along parallel spaced apart lines to effect tearing in accordance with the method of the invention.

In order to provide the necessary energy for tearing the sample 9, a pendulum arm 5'! havinga pendulum weight 59-attached to its outer end is attached to the shaft 51. The structure illustrated in Figs. 2, 3. and 4 has been found particularly satisfactory for interconnecting the shaft 5!, the clamp 21, and the pendulum arm 51-. In the illustrated arrangement, the end of the pendulum arm 57 is attached to the clamp 21 by means of apair of cap screws M or the like (Figs. 3 and 4;) and the shaft 5! is supported within a drilled hole in'the pendulum arm 57-. The shaft 5.! is held in position relative to the clamp 27 and the arm 51 by a pin 63 or the like. (When assembling the rotatable clamp 21 and the pendulum arm 51, the pendulum arm should be disposed at right angles to the plane of the clamping faces of the jaws 35 and 39 as illustrated in Fig. 3.

A stop or support 65 for the pendulum weight 59 is provided on one of the cross braces 31b of the main frame 23. The stop 55 is proportioned and located to support the pendulum weight 59 in a position such that the jaws of the rotatable clamp 21 and of the fixed clamp 25 are parallel and in a position such that the specimen under test will be evenly engaged. When the pendulum weight 59 is supported by the stop 65 the center of mass of the combined rotating system is on a line which extends horizontally outwardly from the axis of rotation ll of the clamp 21.

- The supporting member or stop 65 is movably supported on the frame member 311) so that it can be disengaged from the pendulum weight 59. When the Weight 59 is in position on the stop 65 and the stop 65 is moved to disengage the pendulum, the weight 59 falls downwardly on an are determined by the length of the pendulum arm 51. Angular movement of the pendulum arm 51 rotates the clamp 21, and the torque of the rotating system is applied to the specimen and the known potential energy in the rotating system is available for tearing the test specimen. It is desirable to provide the pendulum weight 59 with a removable projection 91 for engaging the stop 65 in order to permit the renewal of the bearing portion of the pendulum weight after repeated tests. It is also advisable to provide the stop 65 with a detent 69 for engaging the projection Bl on the pendulum weight 59 to reduce the frictional forces between the pendulum and the stop when it is desired to release the pendulum.

In order to indicate the energy required to tear the test specimen, the outer end of the supporting shaft 5| for the rotatable clamp 2'! is provided with a friction collar H to which is attached an indicating pointer 13. The pointer 13 and friction collar H constitute what is commonly called a lazy arm indicator, In order to provide for adjusting the frictional forces between the collar 1| and the shaft 5| an annular groove 15 (Fig. 2) is provided in the shaft 5| and a set screw 11 in the collar H is adapted to compress a spring 18 which in turn biases a plunger 89 into the groove 15. An indicating scale plate or card 19 is attached to one of the depending support members 55 for the shaft, as illustrated at 8| in Figs. 1 and 2. The scale card 19 is divided in suitable graduations 83 (Figs. 1 and 7), according to the units in which the tearing force is to be measured. A knurled adjusting screw 85 is provided in a bracket. M which is supported on the frame 23. The adjusting screw 85 contacts the indicating pointer 13 and provides adjustment of the initial position of the pointer 13. Adjustment of the screw 85 permits compensation to be made for the small amount of friction in the device and insures that the apparatus will operate with extreme accuracy.

In carrying out a test the pendulum weight 59 is raised and is supported upon the stop 65;

this position of the pendulum is illustrated in Figs. 1 and 7. The specimen of paperboard to be tested is cut as shown in Fig. 6 and isinserted in the clamps 25 and 21. The specimen is positioned in the clamps to place the line of the knife cuts midway between the clamps, and to place the horizontal diagonal of the center hole on the axis of rotation of the pendulum arm 51 and of the clamp 21. It should be noted that the center of rotation of the clamp 21 should pass through the test specimen intermediate the plane surfaces ll thereof, and preferably midway between said surfaces (Fig. 6). The pointer 13 is swung in the counterclockwise direction until it is in contact with the adjusting screw 85.

When the detent 69 on the stop 65 is moved to release the pendulum, the pendulum swings downwardly, rotating the rotatable clamp and tearing the specimen. The tear is completed, in the test specimen described, before the pendulum passes through the vertical or lowermost position. During the downward swing and follow through of the pendulum, the pointer remains in contact with the stop, and as the pendulum completes the follow through and begins to swing backwardly, the pointer also swings backward with the pendulum because of the friction between the collar H and the shaft 5! to thereby fix the angular relation between these elements. The pendulum is then returned to its horizontal position'in engagement with the detent and the position of the pointer on the scale is noted. The pointer indicates on the scale the energy loss of the pendulum in effecting the tear in the specimen. It is, of course, first necessary to calibrate rotating system without a specimen in the clamps by permitting the pendulum to swing freely from the detent and then setting the adjusting screw so that the pointer is in zero'position when the pendulum is returned to the detent.

The theory measuring the energy loss in tear ing the test specimen is as follows:

Assuming that m is the mass of the rotating system, that g is the acceleration due to gravity, that 1' is the distance between the axis of rotation and the center of mass of the system, and 00 is the initial angle between the vertical and a line connecting the axis of rotation and the center of mass of the rotating system, the system possesses a potential energy eu equal to the work done in lifting the pendulum from its free vertical position to the initial position. The rela-' tionship between these factors may be expressed as follows:

At the end of the follow through, after tearing the sample, the angular displacement of the pendulum from the vertical is (hand the potential. energy. 61, in that position is equal to:

ei=mgr(l c-os 01) ,(2).

The energy required to tear the sample, e,.isthe' difference between these two values of potential;

For the'instrument described, the angle 00, the

angle in the initial position, is Since cos 90 is zero, Equation 3 for the apparatus becomeszw e=mgr cos 0i (4) The scale card 19 can then be divided in ac cordance with this relationship to indicate teara of the system which is adapted to tear ,a speci-.

men along a line 3.70 inches long, the specimen ranging from 0.010 to-01220 inch in thickness; the rotating-systemweighs 19.8 pounds and the distance between" the. axis of rotation and the center of mass of the rotating system is 6.48 inches. The moment of inertia of this system is 0.261 slug-ftfi. Hence, the initial torque of the rotating system is 1285 pounds-inches or 2056 ounces-inches.

The; testing method and apparatus which has been described in the foregoing meets a long standing demand for an accurate and simple test of the tearing properties of paperboard. Tests in accordance with the invention permit accurate predictions of the tearing resistance of paperboard, and these predictions allow the manufacturer of the containers to recommend the proper container for the material which is to be packaged, thereby permitting the manufacture of adequate containers with a minimum of; cost;

The particular test described in the foregoing determines the resistance of paperboard to continued' tearing, however, if the knife cuts are not made the tes will show the resistance to initial tear. testing the: tearing properties of paperboard will be apparent to one skilled in the art. Various features of the invention which are believed to be new are set forth in the appended claims.

We claim:

1. Apparatus for testing the tearing resistance of paperboard, comprising a frame, a pair of clamps which are adapted to grip the speci men to be torn alcnga pair of spaced-apart parallel lines, said clamps each comprising a pair of opposed jaws, means for rigidly attaching one of said clamps to said frame, means for supporting the other of said clamps for rotation about an axis which passes through said clamps between said jaws whereby portions of each of said jaws are disposed on opposite sidesof said axis-,ysaid axis being normal to the-spaced-a-part lines along which the specimen is gripped, means for rotating said rotatable clamp, and means for measuring theenergy required to effect the rotation of said rotatable clamp.

2. Apparatus for testing the tearing resistance of paperboard, comprising a frame, a pair of clamps which are adapted to grip the specimen to be torn along a pair of spaced-apart parallel lines. eachof said clamps including arnair of opposed, adjustably positionable jaws, means for rigidly attaching, one of said clamps to said frame, means for supporting the other of said clamps: for" rotation about an axis which passes through said .cla'mps between said jaws whereby portions of each of said jaws are disposed on opposite sides. of said axis, said axis being normal to the spaced-apart lines along which the sample is gripped, said jaws being movable toward and away from said axis to dispose the specimen in a position whereby the axis of rotation of said rotatable clamp lies between the faces of said specimen, means for rotating the rotatable clamp to tear the specimen, and means for measuring the energy required to effect the rotation of said rotatable clamp.

3. Apparatus for testing the tearing resistance of paperboard, comprising a frame, a pair of clamps which are adapted to grip the specimen to be torn along a pair-of spaced-apart parallel lines, said clamps each comprising a pair of opposed jaws, means for rigidly attaching one of saidclamps to said frame, means for supporting the other ofsaid clamps for rotation about an axis which passes through said clamps between- Other uses of the apparatus for said jaws whereby portions of each 01" said jaws are disposed on opposite sldes'of said axis, said axis being normal to the spaced-apart lines along which the sample is gripped, a pendulum con-' nected to said rotatable clamp, means for releasably holding said pendulum at a fixed elevated position on one side of its axis thereby to position said rotatable clamp in alignment with the fixed clamp, said pendulum being swingablc' to a determinable elevated position on the other side of its axis thereby to rotate said rotatable clamp to tear said specimen, and means for measuring the difference in potential energy of said pendulum at said fixed and said determinable elevated positions, whereby the energy' required to tear the specimen held in said clamps is determinable.

4. Apparatus for testing the tearing resistance of paperboard, comprising a frame, a pair of clamps adapted to grip the specimen to be torn along a pair of spaced-apart parallel lines, said clamps each comprising a pair of opposed jaws-, means for rigidly attaching one of said clamps to said frame, means for supporting the other of said clamps for rotation on said frame about an axis which passes through said clamps between said jaws whereby portions of each of said jaws are disposed on opposite sides of said axis, said axis being normal to the spaced-apart lines along which the sample is gripped, said rotatable supporting means including a horizontally disposed shaft, a pendulum rigidly attached to said shaft,

means on said frame for releasably holding said pendulum at a fixed elevated position on one side of said shaft thereby to position said rotatable clamp in alignment with the fixed clamp, said pendulum being swingable to adeterminable'elevated position on the other side of said shaft thereby to rotate said rotatable clamp to tear said specimen, and means onsaid shaft for indi cating the potential energy of the rotating system which comprises said rotatable clamp, said pendulum shaft, and said pendulum when said occupies said determinable elevated position whereby the energy required to tear the spec! men is determinable, including a friction collar attached to said shaft, said friction collar carry ing an indicating pointer.

5. Apparatus for testing the tearing resistance of paperboard, comprising a frame, a pair'of' clamps adapted to grip the specimen to be'torn along a pair of spaced-apart parallel lines, said clamps each comprising a pair of opposed'jaws means for rigidly attaching one of said clamps to 14 said frame, means for supporting the otherof' said clamps'for rotation-on said frame about an axis which passes through said clamps between of said jawsare disposed on opposite sides of said axis,'saidj axis being normal to the spaced-apart lines along which the sample is gripped, said rotatable sup-' porting means including a horizontally disposed,-

said jaws whereby portions of each shaft journalled on said frame, a pendulum rigidly attached to said shaft, means on said frame for releasably holding said pendulum atv a' fixed elevated position on one side of said shaft, thereby to position said rotatable clamp in alignment with the fixed clamp to receive said specimen ,therebetween, said pendulum being swingable downwardly from said elevated position thereby to rotate said rotatable clamp to tear said specimen and means on said shaft for indicating the maximum angular displacement of said pendulum from saidelevated position whereby the energy required to tear :the speclw men is determinable, said indicating means including a friction collar on said shaft, a pointer on said friction collar, and means for measuring the angular movement of said pointer.

6. The method of testing paperboard to determine its resistance to tearing generally along a predetermined line, comprising forming a specimen of the paperboard with a cutout portion disposed so as to intercept the central portion of said predetermined line, clamping the specimen along a pair of spaced-apart lines equidistant from and parallel to said predetermined line, rotating the clamped portions of the specimen relative to one another about an axis which passes through said cutout portion and through said specimen between and parallel to its outer surfaces, said axis being normal to said spacedapart lines, whereby said specimen is torn simultaneously from both ends of said predetermined line and generally therealong to said cutout 16 portion, and measuring the energy required to effect the rotation and the tearing of the clamped portions of the specimen.

ROBERT C. MCKEE, GEORGE R. SEARS.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 155,051. Thurston Sept. 15, 1874 1,298,138 Witham Mar. 25, 1919 1,423,841 Elmendori July 25, 1922 1,447,185 Sammet Mar. 6, 1923 1,884,388 Thwing Oct. 25, 1932 1,920,039 Thwing July 25, 1933 FOREIGN PATENTS Number Country Date 96,334 Germany Feb, 25, 1898 

